There are many challenges in the topical application of pharmaceutically active agents. One major objective is to achieve percutaneous penetration of the active agent to the site of treatment. The composition should also have desirable cosmetic characteristics. Application should be easy, smooth, and should not leave a noticeable residue on the surface of the skin. Moreover, the composition should not cause irritation, discomfort, or inconvenience.
Many antifungal and antibacterial agents are used topically to treat epidermal infections. Some antibiotics, such as tetracycline and clindamycin, are also used to treat acne and other skin diseases that are caused, directly or indirectly, by bacteria. One of the side-effects of systemically administered clindamycin is colitis, which can be dangerous and even fatal. Thus, in treating acne, it is desirable to administer clindamycin topically. Cleocin T®, manufactured by Pharmacia-Upjohn, contains clindamycin phosphate, which is inactive in vitro, but is hydrolyzed in vivo to the antibacterially active clindamycin. Cleocin T® is currently available as a gel, a lotion, and a topical solution, and is used for topical treatment of acne vulgaris. 
Lotion and gel topical dosage forms have the disadvantage of extended rub-in and may leave oily residues. The solution form readily runs off the site of application, and therefore it is difficult to apply controlled amounts using the solution form.
The present invention overcomes these disadvantages by providing a composition having at least one pharmaceutically active compound, which is useful for topical administration as described herein, as a foam that is a non-runny, easy to apply, and uses a low residue vehicle. When the foam is applied, body heat causes the foam structure to break down and deposit the active ingredient(s) in the form of a vehicle resembling a solution. The foam composition provides good control of the application of a small amount of product to the desired area.